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Farmers in the inland valleys of northern Ghana are challenged with nitrogen (N) deficiency as a major production constraint of rainfed lowland rice ( Oryza sativa L.). With extremely low use of external inputs, there is a need to efficiently use the systems' internal resources such as native soil N. Largest soil nitrate-N losses are expected to occur during the transition between the dry and wet season (DWT) when the soil aeration status changes from aerobic to anaerobic conditions. Technical options avoiding the build-up of nitrate are expected to reduce N losses and may thus enhance the yield of rice. A field study in the moist savanna zone of Ghana assessed the in situ mineralization of native soil N, the contribution of nitrate to the valley bottom by sub-surface flow from adjacent slopes, and the effects of crop and land management options during DWT on seasonal soil Nmin dynamics and the yield of lowland rice. Large amounts of nitrate accumulated during DWT with a peak of 58 kg ha−1 in lowland soils, of which 32 kg ha−1 were contributed from the adjacent upland slope. Most of this nitrate disappeared at the onset of the wet season, possibly by leaching and denitrification upon soil flooding. While the incorporation of rice straw (temporary immobilization of soil N in the microbial biomass) had little effect on soil N conservation, growing a crop during DWT conserved 22-27 kg of soil N ha−1 in the biomass and Crotalaria juncea supplied an additional 43 kg N ha−1 from biological N2 fixation. Farmers' practice of bare fallow during DWT resulted in the lowest rice grain yield that increased from 1.3 (2.2) to 3.9 t ha−1 in case of the transition-season legume. Growing a pre-rice legume during DWT appears a promising option to manage N and increase lowland rice yields in the inland valleys of northern Ghana. [ABSTRACT FROM AUTHOR]

Risk efficiency of rice grain yield and returns to farm operators' household resources generated from an improved short-duration cover crop fallow system were compared with (traditional) natural bush fallow, and continuous rice-cropping systems. The improved fallow system involved maintainingCalopogonium mucunoides, seeded into a natural bush fallow for 2 years before planting to rice. With no chemical fertilizer application, which reflects farmers' practice in the area, average grain yield for continuous rice (1,185 kg/ha) and the cropping sequence incorporating a natural bush fallow (1,175 kg/ha) did not differ, but were higher for the improved fallow system (1,304 kg/ha). This suggests that nutrient contribution from the leguminous cover crop made up for critical crop N requirements in the improved fallow. Stochastic dominance of grain yield distributions from the improved fallow system, relative to the other two cropping systems, was more dramatic with no N fertilizer application compared to treatments with 30 kg/ha N. Average returns were highest for the improved fallow system, followed by the natural bush fallow-cropping system, and then continuous rice, under the no N fertilizer treatment regime. With 30 kg/ha N fertilizer, income risk efficiency was less clear (compared to treatments with no N fertilizer), especially between continuous rice and the improved fallow treatment, because of faster N mineralization effects on continuous rice. In contrast, the improved cover crop fallow system completely dominated the natural bush fallow treatment under both fertilizer regimes. Rice production systems that incorporated the leguminous cover crop fallow were superior to the natural bush fallow system, based on both grain yield and average farm income risk-efficiency considerations. [ABSTRACT FROM AUTHOR]